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Matter isn't pinned to the space it's in (source: try walking around). As space expands, the other forces which are orders of magnitude stronger than the expans

Matter isn't pinned to the space it's in (source: try walking around). As space expands, the other forces which are orders of magnitude stronger than the expansion of space slide matter along so that distances don't change. You can only detect the expansion of space by measuring the distance between things that are so spread apart that the other forces between them are essentially zero.

CGMthrowaway 1y ago
That's an interesting point. The expansion of space still impacts matter on an atomic level though. The space between atom core and electrons influences its bonding abilities and other properties Analogy, take two attracted magnets, or two opposing electrodes, and expand the space between them. Things change
- spauldo 1y ago
  The magnet analogy works here, so let's give it a try. Put one magnet on a table and then put one under the table so that it's held up by the magnet above it. Then observe what happens. This will take a while, so maybe go get a 2^128 cups of coffee. The space between the magnets is expanding, just like space everywhere. Assuming the table and magnets are immune to deterioration over time, you can come back after several billion years and the distance between the magnets will have stayed the same. Space expanded, sure, but the stuff occupying the space didn't. The forces that hold an atom together are significantly stronger than what's holding the two magnets in the example above.
maaaaattttt 1y ago
That's what I understood from the explanation on stackexchange. But given what you said, if we take the plank length as the shortest length unit, and we consider two theoretical "objects" placed at one plank length away from each other. Does the universe expanding for these two objects mean: 1. the plank length is becomming bigger, 2. more plank lengths are added in between the two objects, 3. Something else and I'm completely off
- mr_mitm 1y ago
  It's 3), I'm afraid. There is no shortest length unit (as far as we know), the Planck length is a constant, and its only significance is that at these length scales, we need a theory of quantum gravity to describe what's happening.
- spauldo 1y ago
  The Plank length thing is just going to add confusion here. Quantum stuff is weird. That's where analogies go to die. But generally speaking, the answer is 2. That's assuming there's no forces between the two objects. Space doesn't expand like the outside of a balloon or a rubber sheet - I hate those analogies because they give you the wrong idea. I'm not aware of any major current theories that say space is quantized, or any theories that have a way of pinpointing a "piece" of space, so the following analogy is flawed. But it should at least point you in the right direction. Draw a line and put eight dots on it. Draw arrows pointing to the fourth, fifth, and eighth dot. We'll call those dots A, B, and C respectively. We're going to measure distance by dots. Dots A and B have a distance of 1. Dots A and C have a distance of 4. Now for each dot, add a new dot before and after it. Measure the distances. Dots A and B now have a distance of 3. Dots A and C now have a distance of 12. Repeat. Measure the distances. Repeat again. Measure the distances. You'll notice that the distances between the dots you've marked is increasing faster with each repetition, and that the distance between A and C is increasing faster than A and B. An object at any of those points would not be experiencing any force - nothing is pushing or pulling on them - but an observer at any of those points would observe the objects at the other points to be accelerating away from them. That's sort like how space expands. Of course, space doesn't have "points" as far as we can tell, so there's all kinds of problems with the above analogy, but hopefully it helps.
eggn00dles 1y ago
Matter is very much pinned to the space its in. If the space between two galaxies expands, the distance between the two galaxies grows. If matter wasnt pinned the distance between the two galaxies would remain the same despite the expansion. Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. See the river model of general relativity for a more thorough explanation.
- spwa4 1y ago
  I think the point is that the force of gravity is so much stronger than the expansion that the (equivalent) force a normal-sized star exerts on your body from the other side of our galaxy is greater than the force expanding space between you and that star. One of them, not all of them together. All of them together are so much stronger it's not even funny. And that's for the "underdense" region that we are in. Not a void, but about half of our galaxy's environment does count as a void.
- rantallion 1y ago
  > Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. Gravity wells aren't pulling surrounding space toward their centre. They're only pulling other masses that occupy the surrounding space.
- spauldo 1y ago
  You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in. Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter isn't pinned to space - it just slides through it. Gravity is too weak to affect distant objects, so we see the effects of the universe's expansion when we look at them.
  - eggn00dles 1y ago
    i walked around and it involved force and energy. i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in. you talk about gravity as if its something distinct from what is driving the expansion of the universe. we have that model, its what newton proposed.

CGMthrowaway 1y ago

That's an interesting point. The expansion of space still impacts matter on an atomic level though. The space between atom core and electrons influences its bonding abilities and other properties Analogy, take two attracted magnets, or two opposing electrodes, and expand the space between them. Things change

spauldo 1y ago
The magnet analogy works here, so let's give it a try. Put one magnet on a table and then put one under the table so that it's held up by the magnet above it. Then observe what happens. This will take a while, so maybe go get a 2^128 cups of coffee. The space between the magnets is expanding, just like space everywhere. Assuming the table and magnets are immune to deterioration over time, you can come back after several billion years and the distance between the magnets will have stayed the same. Space expanded, sure, but the stuff occupying the space didn't. The forces that hold an atom together are significantly stronger than what's holding the two magnets in the example above.

maaaaattttt 1y ago

That's what I understood from the explanation on stackexchange. But given what you said, if we take the plank length as the shortest length unit, and we consider two theoretical "objects" placed at one plank length away from each other. Does the universe expanding for these two objects mean: 1. the plank length is becomming bigger, 2. more plank lengths are added in between the two objects, 3. Something else and I'm completely off

mr_mitm 1y ago
It's 3), I'm afraid. There is no shortest length unit (as far as we know), the Planck length is a constant, and its only significance is that at these length scales, we need a theory of quantum gravity to describe what's happening.
spauldo 1y ago
The Plank length thing is just going to add confusion here. Quantum stuff is weird. That's where analogies go to die. But generally speaking, the answer is 2. That's assuming there's no forces between the two objects. Space doesn't expand like the outside of a balloon or a rubber sheet - I hate those analogies because they give you the wrong idea. I'm not aware of any major current theories that say space is quantized, or any theories that have a way of pinpointing a "piece" of space, so the following analogy is flawed. But it should at least point you in the right direction. Draw a line and put eight dots on it. Draw arrows pointing to the fourth, fifth, and eighth dot. We'll call those dots A, B, and C respectively. We're going to measure distance by dots. Dots A and B have a distance of 1. Dots A and C have a distance of 4. Now for each dot, add a new dot before and after it. Measure the distances. Dots A and B now have a distance of 3. Dots A and C now have a distance of 12. Repeat. Measure the distances. Repeat again. Measure the distances. You'll notice that the distances between the dots you've marked is increasing faster with each repetition, and that the distance between A and C is increasing faster than A and B. An object at any of those points would not be experiencing any force - nothing is pushing or pulling on them - but an observer at any of those points would observe the objects at the other points to be accelerating away from them. That's sort like how space expands. Of course, space doesn't have "points" as far as we can tell, so there's all kinds of problems with the above analogy, but hopefully it helps.

eggn00dles 1y ago

Matter is very much pinned to the space its in. If the space between two galaxies expands, the distance between the two galaxies grows. If matter wasnt pinned the distance between the two galaxies would remain the same despite the expansion. Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. See the river model of general relativity for a more thorough explanation.

spwa4 1y ago
I think the point is that the force of gravity is so much stronger than the expansion that the (equivalent) force a normal-sized star exerts on your body from the other side of our galaxy is greater than the force expanding space between you and that star. One of them, not all of them together. All of them together are so much stronger it's not even funny. And that's for the "underdense" region that we are in. Not a void, but about half of our galaxy's environment does count as a void.
rantallion 1y ago
> Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. Gravity wells aren't pulling surrounding space toward their centre. They're only pulling other masses that occupy the surrounding space.
spauldo 1y ago
You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in. Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter isn't pinned to space - it just slides through it. Gravity is too weak to affect distant objects, so we see the effects of the universe's expansion when we look at them.
- eggn00dles 1y ago
  i walked around and it involved force and energy. i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in. you talk about gravity as if its something distinct from what is driving the expansion of the universe. we have that model, its what newton proposed.